Throw String in Kotlin/JS leads to ClassCastException

[ilgonmic]

Describe the bug

https://youtrack.jetbrains.com/issue/KT-72174/Kotlin-JS-Coroutines-throwing-JS-string-does-not-stop-the-job

JavaScript allows to throw strings, but it leads to ClassCastException is thrown in kotlinx-coroutines-core
Looks createCauseException is the root cause of ClassCastException , String is not Throwable so it is trying to be cast to ParentJob

kotlinx.coroutines/kotlinx-coroutines-core/common/src/JobSupport.kt

Lines 748 to 752 in 6c6df2b

	// cause is Throwable or ParentJob when cancelChild was invoked
	private fun createCauseException(cause: Any?): Throwable = when (cause) {
	is Throwable? -> cause ?: defaultCancellationException()
	else -> (cause as ParentJob).getChildJobCancellationCause()
	}

Provide a Reproducer

class TestClient2 {
    @Test
    fun testThrowString() = runTest {
        val job = launch {
            println("throwing a JS string")
            throw IllegalStateException("EMPTY") //In JS everything can be thrown, even a String!
            println("not executed....")
        }

        println("waiting for job")
        job.join()
    }
}

[konti-aditi]

Instead of assuming non-Throwable values are ParentJob
We can wrap them in an IllegalStateException with a descriptive message
This maintains type safety while handling JavaScript's flexible throwing behavior
( LMK if this approach is correct I'd love to work on it )

[dkhalanskyjb]

Unfortunately, I see no way of fixing this on our side. To wrap an exception, you first have to catch it somehow, and I didn't succeed in doing that without writing custom js("") blocks, and certainly not in the common code of our multiplatform library. I opened an issue for Kotlin/JS: https://youtrack.jetbrains.com/issue/KT-73710/No-way-to-catch-every-JS-error-in-common-code UPD: catch (e: dynamic) does work in JS without custom js("") blocks, but this, too, doesn't compile in common code.

[dkhalanskyjb]

https://youtrack.jetbrains.com/issue/KT-42465/Provide-a-way-to-catch-everything-in-catch-block-in-common-code states that this is not a supported use case in Kotlin/JS.

[jschneider]

The main topic is:
When calling other JS libraries within a coroutine we can not guarantee, that this library is well behaved. It might just throw a string or something else.
This breaks the coroutine, which is quite bad - especially if using a supervisor job...

[dkhalanskyjb]

@jschneider, I see your point, and I even agree, but this out of scope of kotlinx.coroutines. Today, even runCatching { } won't catch thrown strings, which means almost nothing in the Kotlin ecosystem is prepared for this. If Kotlin/JS decides this is worth supporting, we'll certainly do our part (if it won't start working automatically, which it may; in my opinion, it is strictly the responsibility of the compiler to wrap every throwable into a Throwable).

If you want to challenge the decision, it's best to share your thoughts under https://youtrack.jetbrains.com/issue/KT-42465/Provide-a-way-to-catch-everything-in-catch-block-in-common-code